It is known to produce relatively thick slabs of synthetic resin material and, especially, thermoplastic material, by extruding the material in a relatively thick and flat strand from a so-called broad nozzle attached to an extruder. The slab can then be calibrated between a pair of rolls of a roller assembly during which the slabs can be cooled or heated for solidification or tempering purposes.
When slabs are manufactured in this manner with a thickness of 60 millimeters or more, it has been found that they undergo, with time, deformation, bulging or warping which is of significant disadvantage, especially if the slabs are to be used as structural materials and to be reproducible in size, shape and physical properties.